ABSTRACT
In this process, the organic matter in sludge is broken down by microorganisms in the absence of oxygen.
Anaerobic digestion is usually divided into two operational modes, either “high rate” or “low rate.”
In low rate, or one stage digestion, fresh sludge is fed into the digester two or three times daily. As digestion occurs, three distinct layers form in the tank. A scum layer is formed at the top of the digester, a supernatant level below the scum, and a sludge layer at the bottom. The sludge layer also has an actively decomposing top layer and a stabilized bottom layer.
High rate systems may use one or two stages. A two-stage high rate system uses a first stage digester for sludge stabilization, and a second-stage digester for settling and thickening of the decomposed sludge. In a single-stage high rate system, the second stage is replaced by some other thickening process.
Anaerobic digesters are heated to 85 to 95°F and are operated to provide 10 to 20 days detention time for the digesting sludge.
Anaerobic digestion converts about 50 percent of the organic solids to liquid and gas. The gas can be used as fuel, the liquid phase is usually returned to the head of the plant, and the remaining sludge is disposed of.
The process is most successful when fed a high percentage of primary sludge. Waste activated sludge does not settle or dewater well after digestion.
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